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Chromatografy Methods for Environmental - Ando D.J.

Ando D.J. Chromatografy Methods for Environmental - Wiley publishing , 2003. - 265 p.
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124 Methods for Environmental Trace Analysis
DQ 7.8
Comment on the results obtained in this study (see Figure 7.11).
In the first situation, (see Figure 7.11(a)), it is noted that the recovery of organochlorine pesticides is influenced by the soil itself. The highest recoveries are obtained from ‘Celite’, an inert siliceous matrix. This indicates that SFE suffers from some matrix dependency. This behaviour is further investigated in the second situation (Figure 7.11(b)). In this case, the matrix dependency is found to be soil organic matter (SOM) related. It is found that the highest recoveries of organochlorine pesticides are obtained when the soil organic matter is low (15% or less) and vice versa.
7.6 Microwave-Assisted Extraction
Microwave-assisted extraction (MAE) utilizes organic solvent and heat to extract organic pollutants from solid matrices. The major difference between this approach and others is the use of a microwave oven as the heat source. For background information on microwave ovens, see Box 5.1.
7.6.1 Instrumentation
At present, a number of manufacturers supply microwave ovens which are specially designed for laboratory use. However, during the initial period to develop microwave-assisted extraction, domestic microwave ovens were often employed in laboratories for this purpose. The dedicated instruments possess several advantages over domestic ovens, particularly in terms of safety features, although they are considerably more expensive. Generally, the dedicated microwave ovens can be divided into either a closed-vessel style or an open-vessel style. The former is typified by the ‘MARS 5’ system, supplied by the CEM Corporation, USA, and the latter by the ‘Soxwave’ system, from Prolabo Ltd, France (now the CEM Corporation). In addition, some laboratory microwave ovens, such as the QLAB 6000 model, developed by the Questron Technologies Corporation, Canada, can be set for closed- or open-vessel operation, or for the flow-through mode. Schematic diagrams of atmospheric (aMAE) and pressurized (pMAE) systems are shown in Figures 5.5 and 5.6, respectively.
The MARS 5 system allows up to 14 extraction vessels (XP-1500 Plus™) to be irradiated simultaneously. In addition, other features include a function for monitoring both pressure and temperature, and most notably, the system is equipped with a solvent alarm to call attention to an unexpected release of flammable and toxic organic solvent. The microwave energy output of this system is 1500 W
at a frequency of 2450 MHz (at 100% power). Pressure (up to 800 psi) is continuously measured (measurements being taken at the rate of 200 s-1), while the temperature (up to 300°C) is monitored for all vessels every 7 s. All of the sample vessels are held in a carousel which is located within the microwave cavity. Each vessel has a vessel body and an inner liner. The liner is made of ‘TFM’ fluoropolymer and has a volume of 100 ml. A patented safety system (AutoVent Plus™) allows venting of excess pressure within each extraction vessel. The system works by lifting of the vessel cap(s) to release excess pressure and then immediately resealing to prevent loss of sample. If solvent leaking from the extraction vessel(s) does occur, a solvent monitoring system will automatically shut-off the magnetron, while still allowing the exhaust fan to continue working.
The Soxwave system possesses a microwave energy output of 300 W at full power. It can operate at percentage power increments from 0-100% and for different time intervals. As with other open-vessel systems, it deals with individual samples sequentially. Its sample vessel is a glass container, which looks like a large boiling/test tube, into which the sample is introduced, followed by the selected solvent. The sample-containing vessel is positioned within a protective glass sheath and is combined with an air or water condenser to prevent loss of volatile analytes and solvent. Commonly, the open-vessel system does not normally have a temperature-monitoring function.
Table 7.2 lists some of the characteristics of the solvents which are most commonly used in MAE.
DQ 7.9
Why is hexane on its own not a good solvent for MAE?
In order for microwave heating to occur, the solvent must have a permanent dipole moment. This can be assessed by considering a solvent’s
Table 7.2 Characteristics of solvents commonly used in microwave-assisted extraction. From Dean, J. R., Extraction Methods for Environmental Analysis, Copyright 1998. © John Wiley & Sons Limited. Reproduced with permission
Solvent Dielectric constant Boiling point (°C) Closed-vessel temperature (°C)a
Acetone 20.7 56.2 164
Acetonitrile 37.5 81.6 194
Dichloromethane 8.93 39.8 140
Hexane 1.89 68.7 —
Hexane-acetone — 52.0b 156
Methanol 32.63 64.7 151
aMeasured at 175 psi. b Determined experimentally.
Methods for Environmental Trace Analysis
Figure 7.12 Typical procedure used for the microwave-assisted extraction of solids.
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